Chemical plating of thermoplastic resins



United States Patent M US. Cl. 11747 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of pretreatiug molded plastics prior to the application of electroless chemical plating. Chemically plated molded plastics maintain good surface condition and brightness, while the plating, thereby greatly increasing the adhesive quality of crazing is generated by immersing the article in a preplating bath containing orthophosphoric acid, an acid phosphate, or an ester plasticizer.

This invention relates to a method of pretreating molded plastics prior to the application of electroless chemical plating, thereby greatly increasing the adhesive quality of the chemically plated layer and resulting in corresponding improvement of the final electroplated article.

Recently plastics, or more particularly thermoplastic resins, have been coated by a chemical plating process by which a thin electrically conductive layer of copper or nickel is deposited after which the article is electroplated in the conventional way with chromium or other suitable metal. This new technique has largely supplanted the previous practice of electroplating die castings of zinc or other metal. The reasons for this change in the art have been the light weight of the resulting article, the simplicity of processing, and the lack of need for surface polishing of the chemically plated plastic.

The electroless chemical plating process is conventionally performed in the following manner. A molded plastic article is immersed in a mixture of chromic acid and sulfuric acid. Heat is then applied to oxidize the surface of the plastic article, and convert its hydrophobic property to hydrophilic. After this step, the article is dipped in an aqueous solution of stannous chloride, acidified with hydrochloric acid, to add a thin film of stannous chloride to its outer surface. Another very brief bath in an aqueous solution of palladium chloride, acidified with hydrochloric acid, follows, and the palladium chloride is reduced by the stannous ions, and a thin layer of metallic palladium, bonded to the surface of the plastic, is thereby deposited. Thereafter, the molded article is immersed in an alkaline aqueous solution of chemical copper, possessing a pH of about 12. This solution, containing Formalin as a reducing agent and Rochelle salt as a complex forming agent, is utilized to wash the plastic, causing free copper ions, which are separated out, to be deposited as metallic copper on the surface of the molded plastic. Instead of copper, nickel is often applied by the electroless plating technique.

When the aforementioned conventional chemical plating method is applied to molded plastics, particularly the three component polymers, such as the ABS resins comprised of acrylonitrile, butadiene and styrene, an adequate degree of adhesion between plating and plastic surface is difiicult to achieve. As a result of the injection and extrusion molding methods, involving rapid heating, rapid cooling, and rapid flow under high temperature and high pressure, the problem of poor adhesion in the plating proc- 3,522,?3 Patented July 28, 1970 ess is ever present. Since ABS resin is generally shaped by the injection or extrusion molding method, the solution to this plating problem is of the utmost importance.

The manufacture of plated products possessing a high degree of adhesion without losing basic brightness has long been a desired achievement. To accomplish this goal, numerous pretreatment procedures have been attempted prior to chemically plating the plastic. Treating the plastic surface with a mixture of chromic acid and aqua regia to corrosively oxidize the surface, annealing techniques, and mechanically roughening the surface of the plastic are a few methods that have been attempted. It has been determined, that although these harsh treatments may increase the degree of adhesion, extensive deformation, degradation, and loss of surface brightness generally accompany these procedures.

In the past, prior to chemical plating, molded plastic articles have also been dipped in certain solvents to produce cracking or crazing without causing dissolution of the molded article. The degree of cracking or crazing is governed by the stress and strain applied to the plastic at the time it is molded, and these surface distortions greatly affect the final chemical plate. After treatment with a solvent, the irregularities in the surface of the plastic molded article will slightly improve the adhesion force of the coating applied by a non-electrolytic chemical plating process; however, the modest improvement in adhesion of the plated surface is far outweighed by the rough surface finish and lack of lustre in the plated prod uct. On the other hand, those plastics which are plated without first being pretreated, display a characteristically smooth and bright finish, but poor adhesion.

The object of the present invention is to improve the poor adhesion of the plated layer effected by a conventional electroless chemical plating procedure, while eliminating the problems of deformation, degradation, and loss of surface brightness. The instant invention produces a chemically plated plastic article which displays a coated layer vastly improved in its adhesive qualities, yet displaying none of the ill effects produced by the old pretreatment techniques. This novel procedure aids in pro ducing a chemical plate superior to one obtained by pretreatments with strong corrosive oxidation, immersion in a solvent, annealing, or roughening the surface mechanically, and brings about an improved adhesion force greater than 2,000 grams per square centimeter, without marring the surface appearance of the plated plastic article.

Applicant has discovered that chemically plated molded plastics maintain good surface condition and brightness, while the plated layer adheres firmly to the molded article only when crazing is generated without cracking the surface prior to the plating process. Applicant has further determined that the aforementioned crazing can be selectively accomplished by immersing the molded plastic article in a pre-plating bath of warm water containing either orthophosphoric acid, an acid phosphate with a pH value lower than 6, or an ester plasticizer. The warm water solvent may be substituted with a lower alcohol of 4 or less carbon atoms, an aliphatic hydrocarbon of 6 to 12 carbon atoms, an aromatic hydrocarbon of 6 to 12 carbon atoms, or an alicylic hydrocarbon of 6 to 12 carbon atoms, Based on his findings, applicant has achieved a method which is capable of producing plated molded plastics displaying excellent surface condition, brightness and firm adhesion of the plated layer.

More specifically, the instant invention relates to a method of pretreating molded thermoplastic resins prior to the application of the chemical plating. Generally of three or more components, the thermoplastic resins, which best respond to applicants method, possess a conjugated diene and a vinyl aromatic compound as necessary constituents, while they are further composed of vinyl cyanide or a monomer of one or more esters of methacrylic acid. The molded thermoplastic resin is immersed in a solution of about parts per million to about 50% by weight of ortho phosphoric acid, an acid phosphate with a pH value less than 6 or an ester type plasticizer in a solvent of either warm water, a lower alcohol of 4 or less carbon atoms, or in an aliphatic, aromatic or alicyclic hydrocarbon of 6 to 12 carbon atoms. Such immersion takes place for about 10 seconds to about 1 hour.

Those thermoplastic resins composed of three or more components, including a conjugated diene and a vinyl aromatic compound as necessary components include, but are not limited to, the so-called ABS resins which may be either (a) physical blends of butadiene-acrylonitrile rubbery copolymer and styrene (which can be partially or entirely replaced with alpha-methyl styrene)- acrylonitrile resinous copolymer or (b) graft polymers of rubbery polybutadiene or rubbery butadiene-styrene copolymer with styrene (which can be replaced partially or entirely with alpha-methyl styrene) and acrylonitrile, with or Without separately formed styrene-acrylonitrile resinous copolymers. The often referred to ABSM resins lend themselves well to applicants pretreatment process and they respond quite favorably. Representative of these ABSM resins are the graft polymers obtained by graft polymerization of a polybutadiene or a copolymer of butadiene and styrene with a mixture of monomers of styrene, acrylonitrile and methyl methacrylate, and also ABSM resins made up of a copolymer of butadiene and methyl methacrylate with a mixture of styrene and acrylonitrile, wherein said styrene may be substituted with alpha-methyl styrene; or mixtures of such graft polymers composed of two or three components prepared from styrene, acrylonitrile and methyl methacrylate. Also included among the thermoplastic resins which are receptive to applicants pre-plating process are the MBS resins which contain butadiene, styrene and methyl methacrylate, and are free of the acrylonitrile component normally found in the ABSM resins.

The pretreatment solution utilized in the process of the present invention is composed of at least two component parts. One component, an additive, constitutes a minor portion of the solution, while the solvent in which the aforementioned additive is dissolved or dispersed, comprises a major proportion of the solution. It is to be noted, that these additives and solvents must not harshly affect the molded plastic article, i.e. they must not dissolve the plastic, but merely selectively generate crazing without cracking the surface. In addition, it is not necessary that the additive be completely soluble in the chosen solvent, thereby forming a true solution, for a good dispersion, in which the additive is uniformly dispersed, may be utilized as well in practising my invention. The dispersion type pretreatment bath may best be illustrated by reference to the mixture of tricresylphosphate in water, which mixture forms an excellent bath for applicants novel process. Therefore, the use of the term solution, referred to herein, will include not only true solutions but also uniform dispersions.

As well as ortho-phosphoric acid, the acid phosphates including, but not limited to, sodium dihydrogen phosphate, potassium dihydrogen phosphate, calcium monohydrogen phosphate, magnesium monohydrogen phosphate and barium monohydrogen phosphate are ideal additives for applicants purpose. Other desirable additives are taken from the group of ester type plasticizers which include phthalate plasticizers, e.g. dibutyl phthalate, dioctyl phthalate, dinonyl phthalate, dodecyl phthalate, dilauryl phthalate, nonyl octyl phthalate, and butyl benzyl phthalate; phosphate plasticizers, e.g. tricresyl phosphate, triphenyl phosphate, trioctyl phosphate, trinonyl phosphate, tridecyl phosphate and tridodecyl phosphate; and succinate plasticizers, e.g. diisododecyl succinate, dilauryl succinate, dibutyl glycol succinate, and dibutoxyethyl succinate.

As a solvent, warm water, the lower alcohols of four or less carbon atoms, and aliphatic, aromatic and alicyclic hydrocarbons of 6 to 12 carbon atoms have been proven most elfective. The lower alcohols of four or less carbon atoms include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, isobutyl alcohol, and tertiary butyl alcohol. N-butyl alcohol may be used, however, it exhibits very little significant effect on the plastic. Those aliphatic hydrocarbons having a carbon atom content of from 6 to 12 are exemplified by hexane, heptane, octane, decane and dodecane. The aromatic hydrocarbons of 6 to 12 carbon atoms, typified by benzene, toluene and xylene are also excellent solvents; while the alicyclic hydrocarbons of from 6 to 12 carbon atoms represented by cyclohexane, cycloheptane, methylcyclohexane, and cyclooctane may also be used.

It should be further noted, that the pretreatment bath may contain more than two components; i.e., each solution may contain two or more additives and/or two or more solvents. The choice of additive and solvent, however, should not constitute a solution which will dissolve, crack, or in any way mar the plastic to be plated. This selection problem is best illustrated when testing certain pretreatment solutions of sodium dihydrogen phosphate, which forms an ideal solution with water, but an inferior pretreatment bath with methyl alcohol. Another minor problem in choosing pretreatment solutions arises when a molded graft polymer of polybutadiene, styrene and acrylonitrile is prepared for plating. Due to its physical make-up, this particular graft polymer responds poorly to any pretreatment bath containing toluene, which tends to dissolve the polymer, and cause cracking on its surface. Under other conditions, however, toluene brings about an excellent plating result, e.g. when used as the pretreatment solvent in a graft polymer containing an excess amount of a copolymer of alpha-methylstyrene and acrylonitrile in combination with styrene and polybutadiene. In this combination the plastic resists the undesirable dissolving and cracking, and only selective crazing is obtained.

The amount of additives to be dissolved or dispersed in the solvent range from 10 parts per million to 50% by weight of the solvent; but 20 parts per million to 20% by weight of the solvent is preferred. The optimum amounts vary with the choice of additive and solvent selected, however when ortho-phosphoric acid or the acid phosphates are chosen, 2 to 3% by weight of the solvent is preferred, while 50 to parts per million is the preferred range for the additive when the ester type plasticizers are utilized.

Generally, those conditions employed during the pre treatment of a molded plastic article are varied with the composition of the thermoplastic resin to be treated and the eventual choice of pretreatment solution. As a rule, when warm water is selected as the solvent, the temperature of the pretreatment bath should be maintained about 5 C. below the softening point of the plastic to be plated. When an organic solvent is utilized, the pretreatment is preferably performed at a temperature ranging from room temperature (25 C.) to about 50 C. The amount of time during which the molded plastic article is allowed to remain in the pretreatment solution may vary from about 10 seconds to about 1 hour, once again depending upon the composition of the article to be treated. The time for treatment is governed by the consistency of the plastic material and its response to the pretreatment solution. Normally, the preferred time to accomplish a desirable degree of crazing is 1 to 2 minutes; however, when toluene is utilized as the solvent in the pretreatment process of an ABS resin containing alpha-methylstyrene, a brief period of 20 seconds has been found to be optimum, immersion for longer than 20 seconds causing the surface of the molded article to become tacky, and unreceptive to the chemical plating.

Applicants novel pretreatment process may also be carried out during the chemical plating procedure if the pretreatment bath is applied between the step of immersing the plastic article in the chromic acid-sulfuric acid mixture and the step of coating the article with stannous chloride.

The present invention will now be further described by reference to the following examples.

EXAMPLE I A plastic test specimen was formulated from a composition comprising 55 parts of a graft copolymer of 50 parts of a polybutadiene rubber graft polymerized with 50 parts of styrene and acrylonitrile (in a proportion of 70/30), and 45 parts of a separately prepared resinous copolymer of styrene and acrylonitrile (in a proportion of 70/30). This composition was molded into a test specimen of a total surface area of 0.5 sq. decimeter by a screw type injection molding machine; maintaining the resin at a temperature of 240 C., at a cycle of 50 seconds, an injection rate of 15 millimeters per second, an injection pressure of 860 kilograms per square centimeter, and a mold temperature of 50 C.

After forming the molded article, conventional electroless chemical plating was applied to the aforementioned test specimen. The plastic specimen was immersed in a mixture of chromic and sulfuric acids, the sulfuric acid being of a concentration of 50% by weight, for 15 minutes at a temperature of 65-75 C. The article was then dipped in a dilute aqueous solution of stannous chloride acidified with hydrochloric acid, for about 5 min., after which it was immersed in a dilute aqueous solution of palladium chloride, also acidified with hydrochloric acid, for about 30 seconds. Lastly, the molded plastic was treated with an alkaline aqueous solution of a copper salt containing a copper ion concentration of approximately 0.5% by weight and a pH value of about 12. This aqueous solution also contained a quantity of Rochelle salt in an amount which produced two times the molar concentration of the copper salt; said lRochelle salt acting as a complex agent. As a reducing agent, the aforementioned copper aqueous solution also contained an excess amount of Formalin. The article to be plated was subjected to this solution for approximately 15 minutes, thereby allowing a film of chemical copper, in a thickness of about 0.3 micron, to form and adhere to the plastic surface. Subsequent to this procedure, another layer of copper was applied through an electrolytic plating process in a normal acidic bath of copper, increasing the thickness of the copper film on the surface of the plastic article to about 40 microns. Finally, a plating of nickel, in a thickness of about 12 microns, and a plating of chromium, in a thickness of about 0.2 micron, were applied.

The plated article produced by the aforementioned procedure, was then tested for adhesion strength and resistance to heating and cooling.

In the standard adhesion test, two linear cuts of millimeters length were made in the plated surface of the molding. Utilizing an Instron tester, the chemical coating was pulled along the linear cut at an angle of 90, at a speed of 100 millimeters per minute, while the force required to accomplish the pulling was recorded on the scale of the Instron tester. This reading was recorded as adhesion strength.

The heating and cooling tests were performed by subjecting the plated article to alternating hot and cold conditions. The test consisted of heating the article at 80 C. for 15 hours, allowing it to cool at room temperature for 30 minutes, and then subjecting it to seven hours of cooling at a temperature of minus C. A 30 minute standing period was followed by careful observation of the plated article. The degree of swelling or loosening which had occurred on the plated surface was determined, while resistance to environmental conditions was also ob- 6 served. Adhesion of the plated molded article which was not pretreated according to applicants process was approximately 550 grams per centimeter, and swelling was observed on the surface after one cycle of heating and cooling.

EXAMPLE II Another molded plastic article was prepared in accordance with the formula set forth in Example I; however, this article was subjected to applicants pretreatment process prior to electrolessly chemically plating its surface. The pretreatment procedure was carried out by immersing the injection molded plastic test specimen in warm water containing 2% by weight of sodium dihydrogen phosphate, maintained at a temperature of C., plus or minus 2 C., for 15 minutes. The pretreated specimen was then subjected to a conventional electrolytic plating process, after which it was observed to possess a smooth, bright finish, and was of superior appearance when compared with that specimen which was not pretreated. An adhesion strength of 2100 grams per centimeter was recorded when this pretreated specimen was tested with the Instron tester, and the same plated molded article displayed no changes after ten repeated cycles of the heating and cooling test.

EXAMPLE III to that used in Example 11. After the pretreatment bath,

conventional electroless chemical plating and electrolytic plating were applied to the plastic specimen, and upon completion of these procedures, the adhesion strength test and heating and cooling tests were performed. These test results were then compared with those results obtained from the testing of an identical plated specimen which was not subjected to applicants pretreatment immersion process. The results were as follows:

Adhesion Heating and Strength, Cooling Tests, gms./cm. Cycles Specimen with Pretreatment Process"... 5, 600 1 10 Specimen without Pretreatment Process. 2, 000 1 10 1 No change.

While the pretreatment in applicants solution caused no difference in the heating and cooling tests, the pretreatment procedure vastly improved the adhesion strength of the plated article.

EXAMPLE IV A test specimen was prepared from the same composition used in Example I, utilizing the injection molding procedure of Example I. The pretreatment immersion process was performed in the following manner: the solution was prepared by adding 0.01 part by weight of tricresyl phosphate to parts by weight of methyl alcohol, and the plastic specimen was immersed therein. Thereafter, the specimen was chemically plated in a manner similar to that performed in Example I, and the adhesion strength and the heating and cooling tests were performed upon the plated article. The test results obtained were then compared with those of a specimen which was chemically plated without first being pretreated by applicants novel process. The following comparison results were recorded:

These results show that by pretreating in applicants immersion bath the adhesion strength and resistance to heat and cold are greatly improved in a plated plastic article.

EXAMPLE V A test specimen was prepared from a composition comprising 35 parts by weight of a graft polymer consisting of 50 parts of a polybutadiene rubber and 50 parts of styrene and acrylonitrile (in a proportion of 70/30) graft polymerized thereon, and 65 parts of a separately prepared resinous copolymer of alpha-methylstyrene and acrylonitrile (in a proportion of 69/31), by the injection molding process. A resin temperature of 270 C. was maintained, and the process was carried out at a cycle of 50 seconds, injection rate of 15 millimeters per second, injection pressure of 860 kilograms per sq. centimeter, and mold temperature of 50 C. The resultant plastic article was subjected to applicants pretreatment immersion in the following manner: the specimen was immersed for 2 seconds in a solution of 0.01 part by Weight of tricresyl phosphate in 100 parts by weight of toluene. The molded article was then removed from the bath, allowed to stand for 30 seconds and was then washed with water to remove the pretreatment solution. Conventional plating was applied as in Example I, ,and the measurement of adhesion strength and heating and cooling tests were performed on the plated surface in the aforementioned manner. The resultant values were then compared with those of a specimen which. did not undergo 1 No change. 2 Change.

The foregoing results disclose great improvement in the plating surface of the article pretreated by applicants process.

EXAMPLE VI A test specimen was prepared from the same composition used in Example V by the compression molding technique. A resin temperature of 190 C., a compression pressure increased gradually to about 100 kilo grams per sq. centimeter in 7 minutes and maintained at that pressure for about 3 minutes were used. The molded test specimen was then subjected to applicants pretreatment process in a manner similar to that executed in Example V. Subsequently, the chemical plating was applied by conventional methods, and the measurement of adhesion strength and heating and cooling values were determined. These test results were then compared with a similar test specimen which was not subjected to the pretreatment process and the following results were obtained:

1 No change.

The results of the aforementioned tests disclose the vast improvement obtained by pretreating a molded plastic article by applicants pretreatment method.

EXAMPLE VII The procedures followed in Examples V and VI were repeated to produce a molded plastic specimen utilized in this test. The test specimens were immersed in a solution containing 2 parts by weight of ortho-phospho-ric acid in 100 parts by weight of toluene for a period of 20 seconds. The specimens were then conventionally chemically plated and were compared on the basis of adhesion strength and temperature resistance. The results of these tests were as follows:

OOMPRESSIONMOLDED Adhesion Heating and Strength, Cooling Tests, gms./cm. Cycles Specimen with Pretreatment process. 2, 000 10 Specimen Without Pretreatment Process 700 10 INIECTIONMOLDED Specimen with Pretreatment Process. 1, 800 10 Specimen without Pretreatment Process. 350 1 1 No change. 2 Change.

Compression molded plastic displays vastly improved adhesion strength in its plated surface, while there is little change in its resistance to heat or cold as compared with an untreated plastic specimen. The same injection molded plastic displays improved adhesion strength and temperature resistance over its non-treated counterpart.

EXAMPLE VIII Using the same composition of Example I, a plastic test specimen was prepared by the injection molding system, following similar temperatures and pressures. The pretreatment and immersion process was carried out in the following manner. This specimen was immersed in warm water of 80 plus or minus 2 (3., containing 50 parts p.p.m. of diisododecyl succinate, for 15 minutes, and then was conventionally chemically plated in a manner similar to the one set forth in Example I. The surface condition of this plated specimen was observed to be smooth and of excellent lustre. Its adhesion strength was determined at 1900 grams per centimeter and no alteration was observed in the coating after 10 cycles of the heating and cooling test.

EXAMPLE IX Once again utilizing the plastic composition of Example I, a test specimen was prepared by injection molding under similar temperatures and pressures. The molded article was then immersed in a solution of chromic acid and sulfuric acid, for 15 minutes, at a temperature of plus or minus 20 C. Following this bath, the article was dipped in a solution of warm water containing 5 parts by weight of ortho-phosphoric acid dissolved in 100 parts by weight of water, maintained at plus or minus 2 C., for 15 minutes. After this pretreatment the remainder of the conventional chemical plating process was carried out on the plastic specimen, and it was then tested for adhesion strength and temperature resistance. An adhesion strength of 2200 grams per centimeter was recorded, the surface condition was noted to be smooth and bright, and no changes were observed in the surface after 10 cycles of the heating and cooling test.

EXAMPLE X A test specimen similar to that of Example IX was once again immersed in a solution of chromic acid and sulfuric acid for 15 minutes at 70 plus or minus 2 C. In place of the solution of ortho-phosphoric acid used in Example IX, a solution of sodium dihydrogen phos phate was substituted. It was prepared by dissolving 2 parts by weight of ortho-phosphoric acid in parts by weight of water adjusted to a pH value of 5 with sodium hydroxide, and the specimen was immersed in this solution for 15 minutes, at a temperature of 80 plus or minus 2 C. When the finished plated specimen was tested for adhesion strength, a figure of 2300 grams per centimeter was recorded. Its surface condition was perfect and no change in its surface was observed after cycles of the heating and cooling test.

EXAMPLE XI A new test specimen was prepared by injection molding a graft polymer comprised of 16 parts by weight of a polybutadiene rubber graft polymerized with 84 parts of a mixture of styrene, acrylonitrile and methylmethacrylate (in a proportion of 58/22/20) under the same conditions set forth in Example I above. The molded article was then immersed in a solution composed of 2 parts by weight of sodium dihydrogen phosphate in 100 parts by weight of warm water, at 80 plus or minus 2 C., for a period of minutes. The aforementioned adhesion strength test was determined after the article was conventionally chemically plated. The values obtained are set forth below and conclusively prove the superior plating obtained with applicants pretreatment procedure.

Adhesion strength, gms./ cm.

Specimen with pretreatment process 900 Specimen without pretreatment process 200 EXAMPLE XII Another test specimen was prepared from the graft polymer used in Example XI by the compression molding technique, under those conditions set forth in Example III. The plastic specimen was immersed in a solution containing 2 parts by weight of sodium dihydrogen phosphate in 100 parts by weight of water at 80 plus or minus 2 C., for 15 minutes; and this pretreated article was then conventionally plated. The adhesion strength was then measured and the result and value compared with that of the specimen which had undergone no pretreatment bath. The following results were obtained:

Adhesion strength, gms./cm.

Specimen with pretreatment process 1200 Specimen without pretreatment process 600 EXAMPLE XIII A three component copolymer of butadiene, styrene and methylmethacrylate, sold under the trademark of Paraloid K-227, was injection molded into a test specimen under the same conditions set forth in Example I. This specimen was then immersed in a solution of 100 p.p.m. of tricresyl phosphate in 100 parts of methyl alcohol, for 2 minutes. The article was then conventionally plated in a manner similar to that set forth in Example I. The adhesion strength was measured, and its value once again compared with that of a specimen having undergone no pretreatment bath. Once again, use of the pretreatment bath improves the adhesive quality of the chemical plating.

Adhesion strength, gms./ cm. Specimen with pretreatment process 700 Specimen without pretreatment process 200 Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. In a process for chemically plating a molding of a thermoplastic resin, which includes a conjugated diene and a vinyl aromatic compound as two of its components, the improvement which comprises immersing the molded plastic in a pre-plating bath comprised of a solvent selected from the group consisting of water, a lower alcohol of 1 to 4 carbon atoms, an aliphatic hydrocarbon of 6 to 12 carbon atoms, an alicyclic hydrocarbon of 6 to 12 carbon atoms and an aromatic hydrocarbon of 6 to 12 carbon atoms and from 10 parts per million to 50 percent by weight of an ester type plasticizer, for a period of time ranging from about 10 seconds to about 1 hour, at a temperature no greater than 5 C. less than the softening point of the plastic to be plated.

2. The process of claim 1, wherein the molded plastic is immersed for about 2 minutes in about parts per million of tricresyl phosphate in methyl alcohol.

3. The process of claim 1, wherein the molded plastic is immersed for about 15 minutes in 50 parts per million of diisododecyl succinate in water.

4. In a process for chemically plating a molding of thermoplastic resin, which includes a conjugated diene and a vinyl aromatic compound as two of its components, the improvement which comprises immersing the molded plastic in a pre-plating bath comprised of about 50 parts per million of an ester plasticizer selected from the group consisting of phthalate plasticizers, phosphate plasticizers, and succinate plasticizers, in water maintained at a temperature about 5 C. less than the softening point of the plastic to be plated, for a period of time ranging from about 10 seconds to one hour.

References Cited UNITED STATES PATENTS 1/1969 Jensen 11747 OTHER REFERENCES ALFRED L. LEAVITT, Primary Examiner T. E. BOKAN, Assistant Examiner US. Cl. X.R. 

